Microstrip and stripline have become increasingly popular transmission line media for short distance, low power applications, including RF microwave circuits and high speed digital circuits. Stripline consists of a printed conductor between two ground planes, typically formed from copper-clad polyethylene sheets. Electrically, stripline has properties similar to coaxial cable transmission lines.
Microstrip is a popular transmission line due to ease of fabrication and circuit assembly. Microstrip has a single ground plane with a dielectric layer sandwiched between a layer of circuit conductors and the ground plane. The circuit conductors are formed by either depositing metal on or etching metal away from the top surface. Generally, microstrip has greater signal attenuation than stripline, and also the power handling capability of microstrip is lower. Stripline provides inherently better crosstalk (i.e., neighboring channel interference) performance because the field within a stripline conductor is totally contained between the two ground planes, whereas microstrip has only a single ground plane. Although crosstalk interference is greater with microstrip, the exposed microstrip conductors are convenient for attaching circuit elements and thus microstrip is typically used for applications requiring the attachment of circuit elements to the board. To reduce crosstalk in a microstrip embodiment, thin signal traces and a thin microstrip board are used.
One common microstrip application is to provide RF signals to transducers of an acousto-optic Bragg cell. In typical applications, the Bragg cell may have 64 or 128, transducers, each responsive to a different RF signal. Because the microstrip conductors are exposed, it is relatively easy to attach the Bragg cell transducers to the microstrip, but crosstalk performance is less than optimum because the microstrip has a conductive ground plane on only one surface. Microstrip can also be used to interface VHSIC (very high-speed integrated circuit) elements and connect VHSIC elements to other devices. Because microstrip is a form of transmission line, it provides the necessary high-speed performance for VHSIC configured devices or any high-speed circuits.
Because stripline offers better crosstalk performance than microstrip, it is desirable to use stripline in lieu of microstrip whenever possible. But since the stripline conductor is sandwiched between the two ground planes, it is difficult to easily access the stripline signal line conductors. One technique for overcoming this difficulty is described and claimed in the commonly assigned U.S. Pat. No. 4,720,690 issued on Jan. 19, 1988, which is hereby incorporated by reference. This patent, entitled Sculptured Stripline Interface Conductor, provides a technique for connecting a circuit element having closely spaced leads to other devices at spaced-apart connections, using a stripline interconnection apparatus. The invention solves the problems of accessing the buried signal lines of the stripline conductor so that a circuit element with closely spaced leads can be attached to one end of the signal line, and spaced-apart connections are available for attaching a connector or other component. The signal lines are closely spaced at the circuit element attach point and spaced farther apart at the connector end.
FIGS. 1 and 2 illustrate a sculptured stripline interface conductor as described in the commonly-assigned patent referenced above. FIG. 1 is a top view and FIG. 2 provides a cross sectional illustration of a sculptured stripline interface conductor 10. Both FIGS. 1 and 2 show a circuit element 18 attached to a signal trace 14. The sculptured stripline interface conductor 10 comprises an upper ground plane 12, a dielectric layer 13, the circuit traces 14, a dielectric layer 15, and a lower ground plane 16. Before the assembly is laminated together, a first hole 22 is cut within the ground plane 12 and the dielectric layer 13. A differently sized hole 24 is cut within the circuit trace layer 14, the dielectric layer 15, and the lower ground plane 16. In FIGS. 1 and 2 the circuit element 18 is a Bragg cell including a plurality of transducers 20 that can be seen clearly in FIG. 1. Because the holes 22 and 24 have different sizes, a portion of the signal trace 14 is exposed for attaching a transducer 20 thereto with a bond wire 25. The point at which the bond wire 25 is attached to the signal trace 14 is designated by reference character 26.
To access the signal line 14 at the periphery of the sculptured stripline interface conductor 10 (i.e., outwardly away from the rectangular holes 22 and 24) additional access holes 53 are cut in the top ground plane 12 and the dielectric layer 13 before lamination. These holes 53 reveal connection pads 30 in the circuit traces 14, providing a connection point for an electrical connection to the transducers 20 via the signal traces 14. Each signal trace 14 begins at a pad 30 and runs to the rectangular holes 22 and 24 where the other end is exposed, as clearly illustrated in FIG. 2. Thus each signal trace 14 is exposed only at the pad 30 where a wire, connector, or circuit element can be attached, and again at the hole 22 where the circuit element 18 is attached. With this invention, the exceptional crosstalk performance of stripline is advantageously available in an application where previously only microstrip could be used. The crosstalk performance is not significantly degraded by exposing the small portions of the signal trace 14 at the pads 30 and in the area of the hole 22.
Construction of the sculptured stripline interface conductor 10, according to the prior art commonly assigned patent, is accomplished as follows. As is well know by those skilled in the art, the upper ground plane 12 and the dielectric layer 13 are commercially available as a single piece. Also, the signal traces 14, the dielectric layer 15, and the lower ground plane 16 are available as a single piece; the signal traces 14 are formed in the upper conductive surface using well-known etching techniques. After the various layers are sculptured, etched, and drilled as required, the two pieces are bonded using a bond film (available from Minnesota Mining and Manufacturing Company; 3-M Center; St. Paul, Minn. 55144) that is inserted between the two pieces. Pressure and heat are then applied so that the bond film laminates the pieces together. In one embodiment this is accomplished by drawing a 14.5 psi vacuum and holding the temperature at 260.degree. F. The assembly is then cooled and the vacuum is released.
The primary disadvantage of this prepregnated film adhesive technique is the requirement that the film be removed in all areas where the signal trace 14 is exposed for connection. This includes the area of the pads 30 and also the area where the signal trace 14 is exposed due to the differently sized holes 22 and 24. Removal of the film adhesive after the structure has been laminated together is a difficult and time consuming process. Typically, it is impossible to completely remove the film adhesive from the sculptured areas. Thus, it is advantageous to utilize a different process, especially in those embodiments where there will be a large number of exposures of the signal trace 14.
It is also extremely difficult to perform selective bonding with the prior art technique. Selective bonding is the process of masking areas that are to be free from adhesive using a liquid tape that can be removed after the adhesive cures. If the prior art film adhesive is used, all the areas to be free of adhesive must be removed from the film, which is a labor-intensive process.
Although the prior art thin film adhesives provide a continuous thickness with known dielectric properties, there are other disadvantages associated with this technique. After the sculptured stripline interface conductor is laminated together, drilling operations through the interface conductor result in delamination around the drilled holes. The delamination is unacceptable and prevents high quality through-plating of these holes.